1. Field of the Invention
The present invention relates generally to an autothermal torrefaction device that is largely self-heated, mobile or stationary, and a process for the torrefaction of biomass.
2. Description of Related Art
Torrefaction is considered to be a pre-treatment technology to make biomass more suitable for co-firing with coal applications. Torrefaction is a thermal process operated between 200° C. and 300° C. in the absence of oxygen and characterized by residence times typically around 1 hour or greater. Torrefaction processes are usually carried out at near atmospheric pressures. Torrefied biomass exhibits increased brittleness, improved grindability which may enable increased co-firing rates in the future.
During the torrefaction of biomass, the biomass partially decomposes and gives off various gases and water vapor, which results in a loss of mass and chemical energy into the gas phase. However, it is well recognized that more mass than energy is lost to the gas phase during torrefaction. This phenomenon results in energy densification. Thus, torrefaction of biomass yields a solid product with lower moisture content and a higher energy content compared to the material in the initial biomass. Specifically, torrefied biomass exhibits numerous desirable properties such as having decreased moisture content, increased calorific values, and a hydrophobic nature. As such, torrefaction has been identified as a feasible approach for improving the properties of biomass such that it is more suitable as a fuel.
However, traditional approaches lack the efficiency to provide a cost effective means for torrefaction of biomass. This inefficiency has hindered the potential commercial applications of the torrefaction of biomass. Previous torrefaction devices have been stationary and dependent upon significant amounts of supplemental heat from external sources, such as power stations, to produce a torrefied biomass. As such, traditional torrefaction devices are not largely self-sufficient in terms of heat energy used for torrefaction.
Another challenge faced by would-be harvesters and sellers of cellulosic biomass, like wood chips, is its low physical and energy density and low value, which makes it economically disadvantageous to ship it more than 30 to 50 miles to a user. Simply stated, previous devices and methods require hauling untreated biomass, which can often include as much as 50% water by weight, from several separate locations to the user's site, where the torrefaction process would take place.
Therefore, there remains a need for a more efficient torrefaction device and process for pre-treating biomass for various users and applications. Further, a need remains for a viable torrefaction device that is mobile and/or modular so that it can be field portable and self-heated to reduce the transportation costs for fuel production and eliminate such costs for carbon sequestration and soil amendment.